Trench type power semiconductor devices such as power MOSFETs are well known. Referring to FIG. 1, a typical power MOSFET 10 includes a plurality of trenches 12 formed in semiconductor body 14. Semiconductor body 14 is usually a silicon die that includes an epitaxially grown silicon layer (epitaxial silicon layer) 16 of one conductivity (e.g. N-type) formed over a silicon substrate 18 of the same conductivity, but of higher concentration of impurities. A channel region 20 (sometimes referred to as body region) is formed in epitaxial layer 16 and extends from the top surface thereof to a first depth. Channel region 20 is of a conductivity opposite to the epitaxial silicon layer 16 (e.g. P-type) and has formed therein source regions 22 of the same conductivity (e.g. N-type) as epitaxial silicon layer 16. As is well known, trenches 12 extend to a depth below depth of channel region 20 and include gate insulation 24, which may be formed with silicon dioxide, on at least the sidewalls of trenches 12. The bottom of each trench 12 is also insulated with silicon dioxide or the like and a gate electrode 26 is disposed within each trench 12. Gate electrodes 26 are typically composed of conductive polysilicon and may be recessed to a position below the top surface of epitaxial silicon layer 16. A typical trench type power MOSFET further includes a source electrode 28 which is electrically connected to source regions 22 and a high conductivity contact region 30 which is also formed in channel region 20. As is well known, high conductivity contact region 30 is highly doped with dopants of the same conductivity as channel region 30 (e.g. P-type) in order to reduce the contact resistance between source contact 28 and channel region 20. A typical power trench type power MOSFET 10 further includes a drain electrode 32 in electrical contact with silicon substrate 18.
Although not illustrated, gate electrodes 26 are electrically connected to a gate runner, or the like, which serves to electrically connect gate electrodes 26 to a gate contact. Thus, the application of an appropriate voltage to the gate contact changes the voltage of the gate electrodes 26. When the voltage of gate electrodes 26 reaches a threshold value (VTH) a channel is formed adjacent each trench 12 in channel region 20, which is of the same conductivity as that of source regions 22 and the region below channel region 20 in epitaxial silicon layer 16 (referred to as drift region). As a result, a current may flow between source electrode 28 and drain electrode 32 of the power MOSFET.
The density of the current that a power MOSFET may accommodate is directly proportional to the number of channels which may be formed per unit area. Thus, the greater the number of trenches per unit area the more current a device can handle. Because of this relationship, designers strive to pack as many trenches 12 as possible for a given die area, which can be accomplished by either reducing the distance between trenches 12 and/or reducing the width of each trench. If the width of trenches 12 is reduced, the width of gate electrodes 26 must also be reduced. However, the reduction of the width of gate electrodes 26 increases the resistance of the gate as a whole, which is undesirable.
FIG. 2 shows an example of a prior art power MOSFET in which gate resistance may be reduced by adding external rims 27 to each gate electrode 26 in order to increase its cross-sectional area. Such a solution, however, increases the overall width of each cell, and thus is not desirable if increasing cell density is desirable.